At present the liquid crystal display (LCD) used in desktop and notebook computers, PDA and Webpad mostly employs a driving device to output high voltage, and through a ceramic transformer or a wired transformer to ignite a cold cathode fluorescent lamp (CCFL). The technique adopted by the conventional driving devices is discussed as follows:
Referring to FIGS. 1A and 1B, an electric unit 14 delivers electricity to a control unit 10 and a driving unit 13. The control unit 10 outputs two positive phase signal waveforms 11 and 12 to control operations of the driving unit 13. The driving unit 13 is a push-pull amplifier consisting of a P-MOSFET 131 and a N-MOSFET 132. When the positive signal phase waveform 11 output from the control unit 10 reaches the P-MOSFET 131, the P-MOSFET 131 is located at the lower side of the positive phase signal waveform 11 and in a conductive state. Another positive phase signal waveform 12 reaches the N-MOSFET 132, the N-MOSFET 132 is located at the upper side of the positive phase signal waveform 12 and in a conductive state. Under the actuation of the P-MOSFET 131 and the N-MOSFET 132, the driving unit 13 outputs a medium voltage level to drive a driving inductor 15 and a ceramic transformer 16 to ignite a CCFL 17. The positive phase signal waveforms 11 and 12 have time difference, namely dead time 111 (as shown in FIG. 1B). The dead time 111 aims to prevent the P-MOSFET 131 and N-MOSFET 132 from being conductive at the same time and resulting in over short and burn out.
The P-MOSFET 131 has characteristics of electronic hole flow while the N-MOSFET 132 has characteristics of electron flow. The electron flow can generate energy three times as the electronic hole flow does. Hence using one N-MOSFET 132 is equivalent to using three P-MOSFETs 131. Therefore employing a high power to drive the ceramic transformer 16 and CCFL 17, the cost of P-MOSFET 131 is much greater. As a result, the driving circuit also is expensive.